This invention relates to methods and apparatus for removing haze from dewaxed hydrocarbon oil mixture boiling in the lubricating oil range. Throughout this specification, it is to be understood that "hydrocarbon oil mixture" and "oil mixture" both mean a hydrocarbon oil mixture boiling in the lubricating oil range.
The problems of wax in lubricating oil are very well known in the art. In the distillation of crude oil, a proportion of wax is present in cuts taken in the lubricating oil range. Some of the wax remains dissolved in the oil, whereas other fractions form a haze as the oil fraction ages at ambient temperatures. Wax in itself is in fact a good lubricant but under comparatively low temperatures such as engine cold start conditions, its presence causes the oil to be thick and viscous and as a result the engine may be hard to turn over at sufficient speed during starting. In addition, wax haze or particulatate matter can lead to plugging of the engine oil filter.Haze manifests itself as a milky or cloudy appearance in the oil and is often caused by wax or by both wax and tiny water droplets being present in the lubricating oil. Typically a minimum of about 0.1% by volume of wax will cause some lubricating oil to look hazy. Therefore the existence of haze caused by the presence of wax crystals or particles detrimentally affects the performance of lubricating oils.
Techniques have been available for many years which enable dissolved wax to be separated from lubricating oil. A well-known approach is to mix an oil solvent with the lubricating oil, chill the mixture to precipitate wax crystals, and separate the wax from the resulting slurry by settling or filtration. "Oil solvent" as used throughout this specification refers to those solvents which when added to an oil mixture result in a lower viscosity for the solvent-oil mixture than for the oil mixture alone. Viscosity reduction is beneficial for enhancing the settling or filtration processes used to separate the precipitated wax from the solvent/oil. Usually, the oil solvent will have the additional property of having a higher solubility for the oil (hydrocarbon oil mixture) than for the wax at any given temperature, so that during chilling of the solvent-oil mixture to precipitate wax, the wax precipitation is enhanced. Liquified hydrocarbon alkanes or alkenes, ketones, toluene or other aliphatics, and light organic chlorides are examples of well known oil solvents. When the wax-laden oil is cooled, then as the temperature of the oil is reduced, different wax fractions start to come out of solution, aided by the oil solvent. Propane is an example of a preferred oil solvent because in addition to its oil solvent properties, it boils or vaporizes at typically encountered conditions of temperature and pressure in propane dewaxing processes, and this in turn leads to an auto-refrigeration effect which can be used to bring about the desired cooling, at least in part, of the wax-laden oil.
The grown wax particles and crystals are separated from the lubricating oil/oil solvent mixture, following the above described propane pretreatment, by means of a suitable mechanical filter. Rotary drum filters are well-known in the art for this purpose. The lubricating oil/oil solvent mixture is brought into contact with a filter cloth, extending around the rotary filter drum, in one angular position of the filter and a wax cake is formed on the filter cloth. In another angular position, the wax cake is washed with a solvent to recover oil from the wax cake and in a further angular position of the rotary filter, wax is scraped from the filter cloth by a scraper or "doctor" blade positioned adjacent the periphery of the rotary filter. The thereby-resulting relatively wax-free cloth then rotates further into contact with the wax-laden mixture of oil and solvent again to collect more wax and this process continues. Since the rate of filtration is directly related to the viscosity of the lube oil/oil solvent mixture, which is lower than that of the lube oil alone, the filtration rate is enhanced.
The wax particles need to be grown as large as possible in the oil solvent pretreatment process, in order to achieve the most effective wax separation by the rotary filters. This requires that the temperature of the wax-laden lubricating oil be reduced very gradually (typically around 2.degree. C. per min). If the temperature were to be reduced more rapidly, much smaller wax crystals would be produced for a given temperature drop. Therefore, the oil solvent pretreatment apparatus has to be specially designed to provide the necessary stringent operating parameters. In addition, dewaxing aids are often used to promote the wax crystallization.
After removal of wax in the rotary drum filter the dewaxed lube oil (DWO) and oil solvent admixture are in general processed in a solvent recovery plant which recovers the oil solvent from the admixture, leaving DWO which is accumulated in oil storage tanks. Generally speaking, the DWO will be found on testing to comply with "specification" requirements but occasionally, due for example to fines breakthrough or malfunction in the dewaxing plant, the lube oil in one of the storage tanks may be determined (usually by visual inspection) to be out-of-specification or hazy. "Out-of-specification" lube oil and "hazy" lube oil are used interchangeably in the specification to refer to DWO which contains sufficient haze that it does not comply with prespecified "specification" requirements. Out-of-specification DWO is too valuable to be discarded and so it has to be processed in some way so as to remove sufficient haze therefrom such that it then complies with specification requirements. The present invention is concerned with improvement in reprocessing out-of-specification lube oil for removing haze therefrom.